The Concept of Classloaders

Classloaders, as the name suggests, are responsible for loading classes within the JVM. Before your class can be executed or accessed, it must become available via a classloader. Given a class name, a classloader locates the class and loads it into the JVM. Classloaders are Java classes themselves. This brings the question: if classloaders are Java classes themselves, who or what loads them?

When you execute a Java program (i.e., by typing java at a command prompt), it executes and launches a native Java launcher. By native, I mean native to your current platform and environment. This native Java launcher contains a classloader called the bootstrap classloader. This bootstrap classloader is native to your environment and is not written in Java. The main function of the bootstrap classloader is to load the core Java classes.


Figure 1; Classloader delegation hierarchy

The JVM implements two other classloaders by default. The bootstrap classloader loads the extension and application classloaders into memory. Both are written in Java. As mentioned before, the bootstrap classloader loads the core Java classes (for example, classes from the java.util package). The extension classloader loads classes that extend the core Java classes (e.g., classes from the javax packages, or the classes under the ext directory of your runtime). The application classloader loads the classes that make up your application.

All three default classloaders follow the delegation model. Before a child classloader tries to locate a class, it delegates that task to a parent. When your application requests a particular class, the application classloader delegates this request to the extension classloader, which in turn delegates it to the bootstrap classloader. If the class that you requested is a Java core class, the bootstrap classloader will make the class available to you. However, if it cannot find the class, the request returns to the extension classloader, and from there to the application classloader itself. The idea is that each classloader first looks up to its parent for a particular class. Only if the parent does not have the class does the child classloader try to look it up itself.

In application servers, each separately-deployed web application and EJB gets its own classloader (normally; this is certainly the case in WebLogic). This classloader is derived from the application classloader and is responsible for that particular EJB or web application. (Note that if an application is deployed as an EAR file--a combination of EJB and webapps--it gets one classloader, no more). This new classloader loads all classes that the webapp or EJB require that are not already part the Java core classes or the extension packages. It is also responsible for loading and unloading of classes, a feature missing from the default classloaders. This feature helps in hot deploy of applications.

When WebLogic starts up, it uses the Java-supplied application classloader to load the classes that make up its runtime. It then launches individual classloaders, derived from the Java application classloader, which load the classes for individual applications. The individual classloaders are invisible to the classloaders of the other applications; hence, classes loaded for one particular application will not be seen by another application.


Figure 2: Individual application classloaders

What if you want to make a single class available to all applications? Load it in a top-level classloader. This could be in the classpath of WebLogic. When WebLogic starts, it will automatically load this class in memory using the Java-supplied application classloader. All sub-application classloaders get access to it. However, the negatives of this approach are clear too. First, you lose the capability of hot deploy for this particular class in all individual applications. Second, any change in this class means that the server needs to be restarted, as there is no mechanism for a Java application classloader to reload classes. You will need to weigh in the pros and cons before you take this approach.